How does one align texture maps of Multiple sizes and resolutions on same sphere?
It sounds like your images have some blank space once you've reduced the resolution. If you simply get rid of that space and make the whole image smaller blender should automatically scale the UVs relative to the image size.

What I mean by this is that if your UVs on the full resolution textures go right up to the bounds of the image, if you have a texture at half that resolution and size the UVs should just be scaled down and still go right up to the bounds of the image.

You can see how I mean if you take a lower resolution image that has no empty space and replace a high res image with it.